1. Field of the Invention
The invention relates to the prestressing of wood elements and structures to improve structural performance.
2. Description of the Related Art
A number of wood elements and structures may be prestressed or post-tensioned to improve their structural performance. For example, glulam beams and girders may be post-tensioned to increase their strength, stiffness and ductility or to reduce the amount or quality of wood required. Engineered wood trusses such as king and queen-post trusses rely on post-tensioning forces to keep their structural integrity, and stress-laminated bridges, decks or panels rely on the prestressing forces to increase their load-distribution characteristics thus their strength, stiffness and ductility.
While wood prestressing has many structural advantages, commercial applications of prestressed structural wood have been very slow to take hold. One major reason for this has been the difficulty to maintain prestress forces over the life of the wood structure. Wood structures and elements tend to lose prestress rather quickly over time due to a number of mechanisms particularly: creep in the wood system, creep of the wood in the high stressed areas near the prestress anchors and shrinkage of the wood due to loss of moisture. Loss of prestress or fluctuations of prestress forces are even more pronounced in structures such as bridges where hygro-thermal-mechanical interactions between the wood structure and its environment are very significant. In such structures, e.g. stress-laminated wood decks, current design practices require very high initial prestress forces, and require periodic re-stressing of the structure in service. These necessary requirements are cumbersome and expensive to apply and often turn engineers and designers away from using prestressed wood systems.
As part of on-going United States Department of Agriculture (USDA) and Federal Highway Administration (FHWA) timber bridge initiatives, many modem timber bridge designs have been developed and used in the US. Some of the most popular designs are now referred to as stress-laminated decks or bridges. In these bridges, longitudinal wood or engineered wood laminations, consisting of either solid sawn lumber, glulam girders, LVL girders, or a combination of these are post-tensioned transverse to traffic. The prestress force causes friction to develop between the wood laminations, enhancing the load sharing capacity of the system and causing the behavior of the individual laminations to approach that of a continuous orthotropic plate.
While stress-laminated timber bridges can be cost-effective and relatively easy to assemble, one of their biggest draw-backs is the need to periodically re-tension them in service. Creep in the wood laminations over time or drying of the wood in service can cause significant losses of prestress. According to the AASHTO Guide Specification for Stress-Laminated Decks, the initial prestress p.sub.i applied to the deck should be 2.5 times the minimum required value p to compensate for losses due to creep and relaxation. Also, the AASHTO Guide Specification calls for re-stressing the deck to the same initial level p.sub.i during the second and again between the fifth and eighth weeks after the first laminating.
The serviceability and structural integrity of stress-laminated bridges depend on maintaining minimum levels of prestress over the long-term. Because of insufficient data on long-term prestress losses in service for various wood species, various wood preservatives, and various environments, stress-laminated bridges constructed in the state of Maine and many other states are now being monitored and periodically re-stressed in service as part of regular long-term maintenance and evaluation programs. Department of transportation engineers and maintenance personnel are often not at ease with a bridge design that needs periodic re-stressing. Another source of concern is the durability of the metal stressing systems in use today when embedded inside treated timber. Because of these concerns, widespread use of stress-laminated bridges appears to hinge on developing a stressing system that requires minimum maintenance in service. That is, an ideal stressing system would be one that (1) does not require re-stressing in service and (2) is made of a durable material system that is resistant to corrosion and other long-term environmental degradation.
An early study on prestress loss in stress-laminated wood systems was conducted at Queen's University using small-scale laboratory models post-tensioned using 19 mm Grade 5 steel threaded bars. The test results showed that the prestress loss may be as high as 65% of the initial prestress over the long term. Restressing could however reduce the prestress loss to 45% of the initial prestress. Subsequent restressing did not show any further reduction of prestress loss. About 50% prestress loss was observed in the Herbert Creek Bridge, the first stress-laminated wood bridge deck, constructed in Ontario, Canada. Another laboratory study of prestress loss conducted on a 14 m.times.3 m deck at the University of Wisconsin showed that the long-term prestress loss exceeds 50%.
In the past, two ways have been proposed to reduce prestress losses in stresslam wood bridges. One way may be to install the bridge at a moisture content (MC) below the expected Equilibrium Moisture Content (EMC) for the site. In the state of Maine, for example, the EMC on membrane-covered and paved CCA-treated timber bridges was found to be nearly 19%. Installing a bridge at a MC&lt;19% will cause the wood to expand in service as it reaches its EMC of 19%. The wood expansion may compensate in part for the loss of prestress in the deck. However this method is not entirely reliable because the prestress levels in the bridges become "at the mercy" of uncertain environmental conditions. An extended dry period may cause prestress forces to drop again to dangerously low levels.
Another way to reduce the prestress losses may be to use curved-washer type spring stacks (Belleville springs) in series with the steel prestressing rods or tendons. The idea is that the springs will absorb some of the movements of the wood in service, leading to a more stable prestress force and reduced losses. Belleville spring stacks were installed on one-half of a stress-laminated timber deck constructed in Maine in 1991 to test this concept. The other half of the bridge used steel threaded rods with no Belleville springs. The Belleville spring stacks added considerable cost to the system (nearly $50/steel stressing rod). They were also difficult to handle and they made it difficult to tension the bridge. Long-term monitoring of prestress levels in the deck indicated little difference in prestress between the half of the bridge with the Belleville springs and the other half of the bridge. The lack of effectiveness of the Belleville springs in this application was attributed at least in part to the corrosion of the spring stacks which caused them to partially "lock" in place. Corrosion protection of these sizable spring stacks would be possible but costly.